Noise limiting circuit



m 23,1946. Y a w, FYLER 2,404,626

NOISE LIMITING CIRCUIT Filed 001;. 5, 1942 Fig I.

Fig.2. I Fig.3.

' MIME/M ems l/ALUE OPfRATM/S 8/45 VALUES L/M/TER 5/213 o 2' 4's 3 /b 1 112 SIGIVAL 0/005 VOLMGE Ihven'Eor: Georg F ler, v M

Hus Attorney.

Patented July 23, 1946 George W. Fyler. Stratford, Conn, assignor to General Electric Company, a corporation of N ew York Application October 5, 1942, Serial No. 460,781

-My invention relatesto signal translating systems and has for its'object. to provide an improved control circuit for such systems.

More specifically, my invention relates to a noise limiting circuit particularly adapted for use in radio or television receiving apparatus.

It is an object of my invention to provide an improved noise limiting circuit which effectively discriminates between undesired noise impulses and desired signals impressed on the inputoi a radio or television receivin'g'apparatus and which prevents such impulses from appearing in the output circuits of the apparatus. a

In my copending application S. N. 364,160, filed November 4, 1940, entitled Control circuits, now matured as U. S. Patent 2,298,083, and assigned to the same assignee as the present invention, I have disclosed a noise limiting circuit in which a noise limiting diode is-connected in shunt to a signal-rectifying diode, the noise limiting. diode providing its ownthreshold bias for variations in both carrier and modulation levels by peak rectification-of the voltages appearing across the signal diode. The threshold is maintained by a resistance-capacitance network which has a relatively long time constant.

It is an object of my invention to provide an improvednoise limiting circuit of the shunt diode type in which the threshold bias for the noise limiting diode is obtained from a separate low imped'ance source and'is adjusted in accordance with the magnitude of inward or negative peaks of the modulating wave.

Another object of" my invention is to rovide an improved noise limiting circuit in which the threshold level is maintained above the peaks of the modulating wave', isadjusted in accordance with the carrier level" and the per cent modulation, and is substantially unaffected by repeated impulse noise.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof; may best be understood by reference to the following description taken in connectionwith the accompanying drawing, in which Fig. 1 diagrammatically represents a portion of the circuits of a radio receiving apparatusembodying my invention, and Figs. 2 andv 3 aregraphs to which reference is made for a better understanding of the operationof my invention.

The radio receiver circuitsof Fig. 1 include a detector l which may be, for example, the second detector of a superheterodyne radio: receiver. ,5; nal reproductioneitherbydirect interference with 14 Claims. (Cl. 250-20) h frequency carrier ,2 waves, modulated by d.e-'

sired audio signals, are supplied from any. suitablesignal receiving apparatus,not shown, to the detector input. through theinput transforme I i having a tuned primary winding l2 anda plurality' of secondary windings l3 and I4, said secondary' windings being connected to separate signal translating channel's; -The'secon'dary winding i3supplies the high frequency oscillations to the signal detector HI which is shown to be of the diode type having an anode" 1'5 and a; cathode it. The signal detector circuitextends irom the anode f5 through a diode load resistor 11' and high frequency by-pass capacitor [8 parallel and the secondary winding 13 to the'cathode IS.

The operation of this portion of the circuit is well known to those skilled in theart and will not-be detailed: here. The detector is effective to dmod'ulate the carrier waves impressedupon the transformer II. In" a superheter'odynereceiver these carrier waves will: generally be of an inter-mediate high frequency lower than the carrier frequency ofv the" received signal: The demodulation products developed across the diode load resistor l1 include" audio frequency .components and a direct current component. The audio frequency components are coupled through the coupling capacitor I9 to a succeeding stage of the receiver which may be, for example, a first audio frequency amplifier, not shown. Subsequent stages of the receiver may include further audio amplification'stages and a sound translating device.

As is well known, interfering electrical disturbances of shortduration and'considerable magnitude are often received on thesignal channel along with the desired signal. These may form an undesirable noise background. with the" desired signal or even mask it completely. Such disturbances are generally grouped'under th term impulse noise asdistinguished from shot noise or thermal agitation noise. These disturbances may arise from various causes; For example, within this category are" natural atmospheric static surges; man-made electricaldisturbances, as from high. frequency apparatus; ignition systems', and similar apparatus; and other sharp impulses. The characteristics of thes undesired noise impulses may vary widely depending upon their source. 'They may be of short duration occurring at irregular intervals or they may appear as-repeated' impulse noise. In all cases they have'a de1eterlous'eflect upon the fidelity ofsigcause they are random in nature, 'it is characteristic that when they are superimposed on a given ing on the network l1,

high frequency carrier wave, these noise pulses ordinarily, in the absence of cross modulation in the preceding stages of the receiving apparatus,

occur only as outward modulationpeaks} The I reason for this phenomenon becomes apparent-v when it is realized that in order to occur as an 4 l8 reproduce the modulation envelope. On the peaks of alternate half cycles, the maximum voltage impressed across the diode from the detector I ll is approximately equal to the sum of the maximum diode load 7 voltage on the network ll, 18 and the peak voltage of the applied carrier wave. These two voltages areapproxi'rnately equal and; hence, the peak voltages applied across the diode-20and the network 2l, 23, 24, 21, from the secondar winding 13 and the detector circuit .are approximately equal to twice the peak amplitude'of the carrier wave developed across the winding l3- inward or negative peak of modulation Of alligh frequency carrier wave, an impulse must have.

definite frequency, phase and amplitude chara cteristics, as well as an observable period of duration, so that an=.appreciab1e interval or. the high frequency wave iscanceledbythe impulse. Noise pulses such as described above; being random in nature, do not have these requisites of. definite, ness.. Hence, such pulses ordinarily. do ,not occur, with such regularity as to cancel aportion of the carrier wave and dcnotoccur as inward modulation peaks, but only as outward or additive peaks of modulation.

circuit inthe manner described later,

In order:to limit. the, eiiectof suchnoise pulses. on the modulationvoltagesappearing on the network I'l,.-|8,a noise. suppression network: is provided. .As is disclosedin my aforesaid U. S. Patent. 2,298,083,. this network includes second diode detector 20 reversely connected inparallel This characteristic :of :noise' pulses is used toadvantage in my. noise limiting Impulse noiseof the type discussed above appears as'outward peaks of modulation on the carrier wave and the diode 20 functions as a liniiter; for these, high outward peaks, the threshold levelfor the limiter'being established by the voltage-across capacitor 2 I. When'an undesired noise impulse of greater magnitude than the threshold level' and'ofshort duration is impressed .onithe secondary l3 of .the transformer .H, a lowgimpedance'path for voltages of; either polarity. is

, provided through one. or the other of the two 'di-,

odes since the ,diodes are reversely connected to pass current. through the output load resistor I]. in both directions andv capacitor 2| is of low im-' pedance ,for impulses which are conducted by diode 20. Therefore, sinceno netrectificati'onof the intermediate frequency. noise transient takes place, it isefiectively suppressed producing substantially no effect'on, the audio output circuit suppliedthrough thefc oupling capacitor. l9.

In the operation of .thelimiter thus :Iar described, it is observed that, while effective limit: ing is obtained for a' sharp impulse-noise, in the presence of repeated impulse noise-the capacitor to the signal diode l0 through a capacitor ,2 L- A discharge circuit comprising resistor. 23 and the upperportionof resistor 24 in. series is also con nected across ithecapacitor 2!. In order to control the potential across capacitor 2| an electronv discharge device 21. is provided having its anode connected to'the common point, of resistor 23 and capacitor 2 Land its .cathodeconn'ected to a point of fixedpotential at the lower, end .Qfresistor 24.. Operating potential. for the. anodeof device 21 may. be provided from any suitablesource, such 'as the battery 28;,theresistor .23 serving as; a; loadresistor for the device 21. In selecting the values of the elements of thispqrfiion of, the cir-j cuit,ycapacitor. 2| is made quite large, for exam-1 ple of the order ofl mfd., and resistors 23 andz l have as low 'a value as is. permittedby the type of amplifying tube selected as'thedevice 21. The internal resistance of the diode 2]] inv the currentconducting direction should also be approximate 1y equal to the internal resistance of the signal diode H! for bestresults; ,z

Considering for the moment only those ele-v ments oi the noise limiting network thus far described, it will be observed that the diode 29 is in circuit with the secondary winding 13 and the network i7 18 and that it is poled to pass current. through these elements when conductive inthe opposite direction from current flowing inthe signal diode l6. Thiscircuit fextendsifrmn the anode 25 of the diode ZOthr-oiigh the secondary winding E3 of thetransformer'lLthe network l1, l8 and the network 2|, 23, 24, 21 to the cathode The signal diode IE1 passes current on the peaks of those half-cycles of the applied carrier wave which make. its anodepositive with respect to its cathode and the modulation voltages appear 2| tendsto be charged to a'higher threshold level by afirst impulse so that a greater. portion of a rapidly succeeding impulseis developed across the output resistor l'l before. the diodeZU be comes conductive to limit the passage of further noise. This action is cumulative so that on ,re

peated impulse noise, the limiting circuit, tends to open up to permit the passagegof noise; to subsequent stages of the receiver.

The above-mentioned limitation operates also with certaintypes of signal to: introduce distor, j

tion.. When the limiter biasing capacitor is charged by narrow, widely separated-modulating peaks, suchfas occur, for example, in speech currents, thecharge on thecapacitorzl tends to leak off between peaks. If thelimiter-pperates belowv the modulation level, such decrease of charge on the biasing capacitor tends to limit a succeeding wave of modulating voltage and introduce distortion in the output signal.

Ideal noiselimiting is more closely approached if. the threshold level is maintained a small amount above the signal level andheld at this desired level in the presence of repeated impulse noise. In accordancewith my invention, this is accomplished by utilizing a separate low impedance source to obtain the thresholdcapacitor biasing potential, by varying the threshold level in accordance with the, strength of the car rier wave and theper cent modulation, and by deriv Thef s eparate low; impedance source comprises the secondary winding l4 supplying themodwlated high frequency wave to the, auxiliary detector.3ll, shown as a diode havingthe diode load resistors 3| and 32 connected in series. .The corn-, Id mon point? 34 of load,resistors;3l,.1321 is heldat from appearing on this resistor;

labic voltage variations do occur on resistor d6" ground potential and the resistors are by-passed respectively by the high frequency by-pass capacitors and 36, Resistors 3| and 32 preferably have aboutequal resistance values, forexample 50,000 ohms. In order to obtain acontrol potential, which varies in accordance with the strength of the: carrier, wave, avoltage divider'is connected across the diode loadresistor 3!. This voltage divider comprises resistor 31 and resistor 38, having a variable contact arm 39. Anaudio frequency by-pass capacitor is connected between the variable contact arm 39 and the point, 34 at ground'potential.. Both resistors fi'land 38' are of large ohmic value andthe capacitor 48 also is of a. large capacitance,- for example 1 microfarad. As a result the carrier control voltage network comprising theportion of the resistor 38 between the. contact arm 39 and the point 34 and the capacitor 4il-hasa relativelylong time constant. Hence, the unidirectional potential appearing across the portion of the resistor 38 between the contact arm'3i3 and the point 36 varies directly with the strength of the carrier,

Wave and i unafiectedby modulation voltages appearing on resistor 3|. 5

The direct current component of the demodulated signals appearing on resistor.- Sl may be utilized for automatic gai control. Unidirectional potentials, negative with respect to ground, are developed atthe left-hand end of resistor 3| and applied'through a low pass filter comprising: resistor 4! and capacitor 42 to the gain control connections of precedin stages. of the receiver, not shown, Arrangement and. operation ,of'thegain control connection willreadil b understood by those skilled in the art withoutgelaboration. .{The potentials are applied to control electrodesof one or more-ofthe tubes preceding. the, detector to vary the receiver gain in accordance with the strength of the received signals, as is wellunderstood in the art.

In,controliing the thresholdbias of the noise limiter diode 2 6 in accordance with the per cent modulation of the highfrequencycarrier waves impressed across the detector l0, it.-,is desirable to take .advantage' of the factthat-impulse, noisepeaks occur ordinarily only as'outward module-'- tion-ipeaks for the reasonsdiscussed above and to obtain a control potential from the inward peaks of modulation. -In accordance with my invention, a modulation controlv potential is obtained from the unidirectionalpotentials appearing across the diode load resistor 32;. Th potenin accordance with the 7 negative modulation amplitudes. The capacitor 4? by -passes the resistor to prevent audio frequency voltages However, sylin accordance with the negative modulation peak The values of the resistor 45 and the. capacitor sistor 48 completes thecircuitof jthe.diodepeak rectifier 45.

The potential appearingacrossgthe resistor 46 is" adjustably supplied by means of the variable contact. arm 50' tothecontroLelectrode 5| of the controlidischarge device, 21. Thecathode 53 of the discharge device 211' is directly connected to ground and the anode is connected through the.

loadresistor 2317 the: source of-operatin-g' potential shown as the, battery 28. The anode 54 is likewise directly connected-to the upper plate of j anode; potential is applied to the control device 21, The device 2'! draws anode current through the. resistor;23 producing a certain potential on the upperplate of the capacitor 2| and thecathode 26, The potential of the lower plate f the capacitor 2] j is now adjusted by variation of the contact arm 55.:on the voltage divider-.24 to obtain a small biasingpotential on the capacitor 2! such that the cathode 26 is slightly positive with respect to the anode 25. In Fig. 2, this slight bias potential: is shown by the portionfie of the curve at the left of the figure. 7

Assume. now: that any unmodulated high frequency carrier waveis impressed upon the re- A voltage is now developed 7 at ground-potential fo alternating current. The

the point 34, varies in accordance with thestrength of the carrierwave impressed upon the resistor '66. to control electrode 5!. and is'efiecti ve v capacitor El-I'tends to be charged through the noise diodelil duetothe peak rectification action oithelnoise limitin'g circuit as previously described. If the carrier wave is represented by the.

curve 6| in Fig. 2, the line 62 represents the bias potential of the cathode 25 due to the action of devices 39 and 21 in conjunction with the sepaondary winding I4 and its associated circuit. As pointed out previously, the negative unidirectional potential, developed across the portion of the resistor 33 betweenthe contact arm 39 and receiver, audio and sylla'bie modulating. waves being removed therefrom by the long time constant circuit comprising. the resistor 38 and the capacitor 4 0/ This carriercontrol potential is' supplied by means of the contactarm 39 and to reduce the anode current drawn by the device 21. The reduction of, the anode current of the device 2 1 results in a decrease in the potential drop. across resistor 23 and an. equal increase in.

the potential across the capacitor 21, the potential of the lower plate of the capacitor 2|, of course, being determined by the position of the contactarm 55. It is apparent that the potential of thecathode 26, shown by thecurve '62, is maintained above the maximum valueof the wave 6| appearing across the diode 20.-

In order that the diode .20 will not operate to limit very weak signalsgit is desirable that the potential across the threshold bias capacitor 2| be raised to a value indicated by the curve 63.,

This is accomplished by adjustment of the conthe curves mud a3 is impulse noise peaks, such as the peaks 65.

V are rectified by the diode 65, audio frequency components are removed by'the by-pass capaci-' tween'curves I and H and curves l2 and .13 may Assume now thatthe carrier wave 6| is modu lated by a signal fi twhich includes a plurality of 1 AS is pointed out above; the noisepeaks 65 ordinarily occur only as outward or additive peaks-of modulation; Accordingly, in my'noise limiting circuitt'he'inward or negative modulation peaks of the 'signal 64 are rectified and the threshold bias B3. is adjusted in accordance with the recti- Y fication voltages thus produced. As the result of such a system, the threshold bias 63 is substantially unaffected by the presence of the noise peaks 65. Instead of being reproduced in the output circuits of the receiver,'the noise voltages are reduced toa low value by the short-circuiting action of the' two 'reversely connected'diodes i0 and 20, which action appears as a load of large magnitude in circuit of the primary winding l2. In order to obtain desirable operating characteristics for the noise limiter circuit, the secondary windings are closely coupled to each other and are critically coupled to the primary winding I 2, so that the 'short-circuiting action ofthe diodes l0 and'20 on the winding 13 appears also in circuit of the winding l4. 1 7

The potential variations of the point 43 are represent a" desirable safety factor which provides acomplete absence of distortion for sudden transients in the 'received signals; Since the threshold bias potential is obtained from a source which isindependent of the signal detector l0 and does not rely upon the self-rectification action of the diode 20, it-is possible to obtain a comness of the noise suppression circuit is material- 2 1y increased by employing wide band interme-.

diate frequency circuits preceding the detector and limiter and relatively narrow band audio circuits following" them. This fact becomes apparent froma study of impulse excitation characteristics of tuned circuits. 7 In' a wide band circuit the train of oscillations, set up bysingle short sharp impulse, has a very high decrement whereas in a narrow band circuit, the'converse is true. In other words, the lengthof the-decay train is inversely proportional to'the acceptance I band width of the system.- On the other'handit similar to those represented by the curve 64 of Fig. 2,the potential comprising a unidirectional component'due to the rectification of the carrier -wave plus a modulating Wave component. The

capacitor; 44 removes the unidirectional componentjleavingonl'y the'modulation wave, such as the wave '54. The negative peaks of the wave 64' j tor 41' and the syllabic voltage variations are re produced on the resistor 46 by the negative moduv A portion of this negative voltagej t lation peaks, issupplied by means of contact arm 5il to the control electrode 5| of the device 21 and is effective to reduce still further the anode current flowing through the device 21 and the potential j vent limiting of a carrier wave. This value is a minimum value for the threshold bias that corresponds to the unmodulated conditions as dis- 1 cussed in connection with cu ves 51; 62, 63 of Fig, 2. Curve H illustrates the manner in which the potential supplied by the contact arm 39 to the control electrode 51 is effective to raise the threshold bias for difierent carrier levels. By adhas been found that the peak amplitude of the train is almost directly proportional'tothe band width; Therefore, it can be shown that the average value of a given impulse train is practically independent of bandwidth. T l 1 From the above principles, it follows that sharp noise impulses impressed on the wide band iritermediate frequency amplifiers preceding 'the apparatus embodying my invention. These valdetector l0 remain sharp and of short duration in transmission to those circuits. If these impulses are now limited substantially to the peak carrier level in the manner previously described,

very little energy remains and; if they are furplitude far below the signal level. V r

Merely for purposes of illustration, the following-datais given for a particular radio receiving 'ues were found to give satisfactory results in a justment of the position of contact arm'39 on the 7 1 resistor :38, the curve, H may be made to occur aboveor below curve 70. Curves I2 and 13illus- I traterespectively the increased biasjvalues and thefoperating bias values for a 30% modulation wens-get we h it o fw e 13 l 7 37 with respect tothe optimum-curve 1.2 jcan beadjusted by, varyingthe'position of contact arm 50 i megacycles 8.25 Band width of IF amplifier Band with of IF amplifier t kilocycles .About 300 Capacitors 18, 35, 36

micromicrofarads 50 Capacitors 21, 22 microfarad 1 Capacitor V 44 do .25 Y Capacitors 40, 4'7 do -1 Resistor 23 ohms 10,000 Resistors 17,31, 32 do; About 50,000 Resistor 48 do 150,000 Resistors 37, 38 megohms 1 Resistor 46 do 5 Resistor. 24 a ohms 10,000 Control amplifier 52 Type 6J5 Anode voltage on amplifier 52 I volts.

particular case, although they are not to be regarded as necessarily applicable to allembodiments of my invention. Frequency of IF amplifier Many modifications within the scopeof my invention will doubtless occur to those skilled'in the art. Hence, while I have shown a particular embodiment of my invention, it'will be understood that I do not wish to be limited thereto,

modulated waves, atranslating circuit 'for said waves and means including means responsive to the averagelevel of said waves and means responsive to the amplitude of inward peaks of said modulating signal for preventing the translation of noise impulses by said circuit.

2. In a receiver for signal modulated carrier waves and subject to undesired noise impulses, the combination with means including a threshold bias for interrupting the output of "said receiver upon the occurrence of an undesired impulse, of means for adjusting saidbias and the operation of said interrupting means in accordance with the magnitude of inwardpealrs of said modulating signal.

3. In a receiver for signal modulated carrier waves, the combination of means for interrupting the output of said receiver upon the occurence of a noise impulse therein, means establishing a threshold bias for said interrupting means, and means for adjusting said bias in accordance with the intensity of said wave and the magnitude of inward peaks of said modulating signal.

4. A noise suppression circuit for a receiver of modulated waves comprising, in combination, a first signal channel normally operative for the translation of an oscillation comprising a signal modulated carrier wave and subject to transient noise impulses having a duration shorter than a wave length of said modulating signal, means for disabling said channel when the intensity of said oscillation is greater than a predetermined threshold level and for restoring said channel to operative condition when said intensity is less than said level, and means including a second signal channel for adjusting said level in accordance with the per cent modulation of said wave.

5. A noise suppression circuit for a radio receiver comprising, in combination. a first signal 1 channel normally operative for the translation o an oscillation comprising a signal modulated carrier wave subject to transient noise impulses, means for d sablin said channel when the intensity of said oscillation i greater than a predetermined threshold level and for restoringsaid hannel to operative condition when said intensity is less than said level. and means including a second signal channel for adjusting said level in accordance with both the intensity of said wave and the per cent modulation thereof.

6. A noise su pression circuit for a radio receiver comprising, in combination, a signal channel normally operative for the translation of an oscillation comprising a signal modulated carrier wave subject to transient noise impulses, means for disabling said channel when the intensity oi said oscillation is greater than a predetermined threshold level and for restoring said channel to operative condition when said intensity is less than said level, and means for adjusting said level in accordance with the magnitude of inward modulation peaks of said wave.

'7. In a receiving system for signal modulated I10 carrier waves, a first'signal channel normally operative for the translation of signal voltages, a shunt circuit including a capacitor connected to said channel, a secondsignal channeltoperative to reproduce signal voltages from said waves,

means responsive to the voltages in said'second channel for chargingsaid capacitor to a threshold limiting level, and means for disabling said first channel in response to a transient "noise impulse therein which increases the voltage across said shunt ,circuit to a value greater than said level. l

8. In a receiving system 'for signal modulated carrier Waves, a first ignal channel normally operative for the translation of signal voltages,

a shunt circuit including a capacitor connected to said channel, a second signal channel operative to reproduce signal voltages from said waves, means in said second channel for rectifying said wave and fo'rproducing a voltage varying in accordance with the intensity thereof, means responsiveto the sum of the voltages in said second channel for charging said capacitor to a threshold limiting level, and means for disabling said first channel in response to a transient noise-impulse therein which increases the voltage across said shunt circuit to a value greater than said level. 1

9. In apparatu for receiving a signal modulated carrier wave a noise suppression circuit comprising, a first signal channel normally operative for the translation of said wave, means for disabling said channel when the intensity of said wave is greater than a predetermined threshold level and for restoring said channel to operative condition when said intensity i less than said level, and mean for adjusting said level in accordance with the per cent modulation of said wave, said last means including a second. signal channel and means for rectifying inward modulation peaks of waves in said second channel.

10. In combination with a source of signal modulated high frequencycarrier potentials subject to interfering noi e impulses, a first signal channel comprising a unilaterally conducting dis charge device and an output impedance adapted to have'demodulated signal potentials developed thereon, a second signal channel comprising a unilaterally conducting discharge device, an impedance adapted to have demodulated signal potentials developed thereon, and an impedance adapted to have potentials varying in accordance with the level of the lmmodulated carrier developed thereon, means for interrupting said first channel upon the occurrence of a noise impulse, and means responsive to the potentials developed in said second channel for controlling the operation of said interrupting means.

11. In combination with a source of signal modulated high frequency carrier potentials subject to interfering noise impulses, a first signal channel comprising a unilaterally conducting discharge device and an output impedance adapted to have demodulated signal potentials developed thereon, a second signal channel comprising a impulse, and means responsive to said rectified peaks and said carrier level varying potential for controlling the operation of said interrupting means. I r

12. In areceiving system for signal modulated carrier waves, a -first signal channel normally operative for the translation of signal voltages, a second signal channel comprising means for producing a first voltage varying in accordance with the level of said carrier waves, means for reproducing said modulating signal from said waves, mean connected to said reproducing means for producing a second voltage varying in accordance with syllabic variations of the inward peaks of said modulating signal, said first and second voltages being connected in series in said second channel, and means responsive to the sum of said first and second voltages for establishing detection of said signals, a noise uppression diode reversely connected across said signal diode through a capacitor, a control circuit comprising an input impedance, a signal detection diode and an output impedance network, a voltage divider connected across a portion of said output impedance and adapted-to produce a voltage responsive to the level of said carrier wave, a rectifier diode reversely coupled to saidsignal detection diode and adapted" to rectify inward peaks of said modulating signal, an output impedance of suitable time constant for reproduction, of syllabic voltage variations of said signal connected across said rectifier diode, said syllabic voltage and said carrier responsive voltage being connected in series, an electron discharge device having a control electrode and an output circuit adaptedto charge said capacitor to a threshold limiting level, and means for supplying said series connected voltages to said control electrode, thereby to make said series connected voltages effective to control said threshold level.

14. In radio receiving apparatus for translating signal modulated carrier waves and subject to undesired noise impulses, the combination of a GEORGE W; FYLER. 

